1. Field of the Invention
The present invention relates to a color display device and, more particularly, to a color display device of active matrix type having a color filter provided on a substrate which carries switching elements formed thereon to drive pixel electrodes.
2. Description of Related Art
Development is being accelerated in recent years of color liquid crystal display devices of the type in which thin-film transistors (TFTS) are used as switching elements for driving pixel electrodes. FIG. 5 shows an example of previously proposed color liquid crystal display device. This device has a glass substrate 0 and TFTs integrated on the glass substrate 0 so as to drive pixel electrodes 1. Each TFT has a device area presented by a semiconductor thin film 2 on which patterned and formed is a gate electrode 3 through the intermediary of a gate insulating film. The semiconductor thin film 2 has a source region S and a drain region D. The TFT having the described construction is covered by a first inter-layer insulating film which is denoted by 4. A wired electrode 6 is formed in a predetermined pattern on the first inter-layer insulating film 4 and is electrically connected to the source region S through a contact hole. The wired electrode 6 constitutes part of a signal line. The wired electrode 6 is covered with a second inter-layer insulating film 5. The aforementioned pixel electrode is electrically connected to the drain region D via contact holes formed in the second and first inter-layer insulating films 5, 4. A color filter 9 is formed so as to color the pixel electrode 1 in three primary colors of R, G and B. The color filter 9 is finely divided into tiny segments each of which is interposed between each pixel electrode 1 and the second inter-layer insulating film 5. Thus, the color filter 9 has red color segments 9R, green color segments 9G and blue color segments 9B. The color filter 9 is formed directly on the glass substrate 0 so as to provide a so-called on-chip color filter structure. The glass substrate 0 carrying the TFTS, pixel electrodes 1 and the color filter 9 will be referred to as "TFT substrate". An opposing substrate 12 is joined to oppose the TFT substrate 0 across a predetermined gap. A counter electrode 11 is formed on the inner surface of the opposing substrate 12. A liquid crystal 13 is held between these substrates 0 and 12.
The above-mentioned on-chip color filter structure is disclosed, for example, in Japanese Patent Laid-Open Nos. 2-54217, 3-237432, 3-72322, 3-119829, 4-253028, 2-153325, 5-5874, and so forth. Structures of the type in which a color filter is provided on the TFT substrate offer various advantages over the structures of the type in which a color filter is provided on the opposing substrate. For instance, no parallax is caused between the color filter 9 and the pixel electrodes 1, because they are superposed one on the other. It is therefore possible to set the pixel aperture ratio to a comparatively large value. The high aperture ratio can be maintained even when the degree of fineness of pixels is enhanced, because mis-alignment between the pixel electrodes 1 and the color filter 9 is significantly reduced.
The structure shown in FIG. 5, however, suffers from the following disadvantage, due to the fact that transparent pixel electrodes 1 are held in direct contact with the color filter 9. Namely, there is a risk that the color filter 9 is damaged by sputtering which is executed for the purpose of forming transparent pixel electrodes from, for example, ITO, tending to cause the surface of the color filter 9 to be roughened. The segments 9R, 9G and 9B of the color filter 9 are isolated at a constant pitch. ITO, when sputtered on the isolation zone, tends to cause imperfect etching during patterning etching conducted to form the pixel electrodes, thus increasing the risk of generation of defects. Furthermore, the pixel electrodes 1 are undesirably stepped due to influence of convexities and concavities of the underlying color filter 9, resulting in defects such as disorder of alignment and disclination of the liquid crystal, as well as generation of reverse tilt domain.
At the same time, impurities contained in the color filter 9 tend to contaminate the liquid crystal 13 and orientation film (not shown) so as to degrade the same and to incur defects such as sticking in dark or bright state.